Introduce clang-format in pre-commit

[chrispap95]

clang-format in pre-commit for C++ files (only src/_ext.cpp for us).

[jpivarski]

The formatted version of _ext.cpp is much easier to read.

That .clang-format file is exactly what I would consider "hard to set up," but having a working model (boost-histogram) helps considerably. I think we'll copy this to format the C++ in Awkward Array. :)

[henryiii]

exactly what I would consider "hard to set up,"

It was basically running -style=llvm, dumping the values to a file, then modifying them ever so slightly to work nicer on pybind11 and to fix one perceived alignment issue (int *x vs int* x).